I think thats about 110 lbs? (living in the last country to not convert to metric lol). That is pretty darn skinny. I'm only 2 inches taller than you and about 45 lbs heavier. Then again I'm sure we have completely different builds. I know, for me, 5'9'' and 150-155 lbs is right where I need to be. I'm sure 5'7'' is probably 135-145 lbs or around that area. So yes, you are considered underweight. But it's all about how you feel.

You say you are 25 and have lost 11 lbs in 5 months without any additional exercise or diet? That is the part that worries me. Have you noticed any other symptoms recently? Particularly I'm wondering if you have been more thirsty than before. More fatigue? Depending on if you have other symptoms, I would start thinking about thyroid issues.

You also mentioned you eat a lot of fast food and fatty meats? I'm assuming crisps are chips lol..... if you are deficient in a vitamin or mineral that is essential to your body, you could be a little malnourished. People who eat crappy foods a lot can still be malnourished. Do you take any multi-vitamins or anything like that?

I would go to your doc, tell them you are worried about your weight loss and see what they say. I would also ask them to do a full blood work up. They will most likely want to test your TSH (thyroid stimulating hormone) and maybe go from there. But I would also ask them to test your B12 levels, Vitamin D and everything else. If it is an issue like that, especially B12, it can escalate quickly. I know a B12 deficiency can cause seizures and all kinds of stuff.

Good luck and keep us posted!